The effect of Nafion ionomer aggregation within the anode catalytic layer for a direct formic acid fuel cell (DFAFC) has been investigated. By simple heat treatment, the aggregation states of Nafion ionomers in aqueous solution can be tuned. Nafion agglomerate sizes in the solution decrease and aggregate size distribution becomes narrow with the increase in heat-treatment temperature. At a heat-treatment temperature of ca. 80 degrees C, nearly monodispersed Nafion ionomers corresponding to an aggregate size of ca. 25 nm in the Solution are observed. The use of small Nafion ionomer agglomerates in the Nafion solution for anode catalytic layer significantly improves the performance of the passive DFAFCs. Impedance analysis indicates that the increased performance of the passive DFAFC with the anode using Nafion solution pretreated at elevated temperatures could be attributed to the decrease in charge-transfer resistance of the anode reaction. The decrease in Nafion aggregation within the catalyst ink leads to an increase in Nafion ionomer utilization within the catalyst layer and an improvement in catalyst utilization: thus enabling us to decrease Nafion loading within the anode catalytic layer but with slight improvement in DFAFC's performance. (C) 2009 Elsevier B.V. All rights reserved.